1. Field of the Invention
The present disclosure relates to a self-oscillating class D amplifier.
2. Description of the Related Art
Self-oscillating class D amplifiers have been known conventionally as a type of amplifier for power-amplifying signals such as audio signals (for example, refer to JP-B-61-21007).
FIG. 4 shows a circuit example of a conventional self-oscillating class D amplifier. An operational amplifier 42 and a capacitor 43 constitute an integrator for integrating the audio signal input through the input terminal 41 of the amplifier. A comparator 44 compares the output of the operational amplifier 42 with the feedback output from a first feedback circuit described below and outputs a PWM (pulse width modulation) signal. A switch driving circuit 45 drives a switch circuit 46 based on the PWM signal output from the comparator 44.
The switch circuit 46 is configured by two power transistors (for example, FETs) connected to positive and negative power supplies. When driven by the switch driving circuit 45, the switch circuit 46 amplifies electric power of the PWM signal, or in other words power-amplifies the PWM signal and outputs the amplified signal. A low-pass filter (LPF), configured by an inductor 47 and a capacitor 48, removes the carrier component from the PWM signal output from the switch circuit 46 and demodulates (extract) the power-amplified audio signal. Numeral 53 designates an output terminal of the amplifier to be connected to a load, such as a speaker.
The first feedback circuit, configured by a resistor 49 and a capacitor 50, feeds back the output signal of the output terminal 53 to the inverting input terminal of the comparator 44. The loop circuit, configured by the comparator 44, the switch driving circuit 45, the switch circuit 46, the LPF (the inductor 47 and the capacitor 48) and the first feedback circuit (the resistor 49 and the capacitor 50), self-oscillates at a frequency f0 at which the phase delay of the audio signal going around the loop circuit is 180 degrees. The self-oscillation frequency f0 can be adjusted by changing the delay of the first feedback circuit.
A second feedback circuit, configured by resistors 51 and 52, feeds back the output signal of the output terminal 53 to the inverting input terminal of the operational amplifier 42. This feedback route applies negative feedback to the audio band and determines the gain of the amplifier in the audio band.
In the circuit described above referring to FIG. 4, in the case of designing an amplifier having high overall gain and high output power, it is assumed that high voltage fluctuations (for example, ±100 V) may occur in the voltage at the output terminal 53. In such a case, input voltage having fairly large fluctuations is also assumed to be input to an input side of the circuit. Hence, the input voltage applied to the comparator 44 has large fluctuations. Furthermore, since the amplifier is a self-oscillation type, a feedback signal for self-oscillation fed back from the output side is applied to the input signal of the comparator 44. As a result, there is a problem that a semiconductor device having a fairly high breakdown voltage is required to be used as the comparator 44.